title: When and where does mortality occur in migratory birds? Direct evidence from long-term satellite tracking of raptors.
abstract: Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).
authors.affiliation: Department of Biology, Lund University, Ecology Building, SE-22362, Lund, Sweden; Dutch Montagu's Harrier Foundation, PO Box 46, 9679 ZG, Scheemda, The Netherlands; Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 11103, 9700 CC, Groningen, The Netherlands.
authors.forename: Raymond H G
authors.initials: RH
authors.lastname: Klaassen
authors.forename: Mikael
authors.initials: M
authors.lastname: Hake
authors.forename: Roine
authors.initials: R
authors.lastname: Strandberg
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Koks
authors.forename: Christiane
authors.initials: C
authors.lastname: Trierweiler
authors.forename: Klaus-Michael
authors.initials: KM
authors.lastname: Exo
authors.forename: Franz
authors.initials: F
authors.lastname: Bairlein
authors.forename: Thomas
authors.initials: T
authors.lastname: Alerstam
mesh_terms: D000823:Animal Identification Systems; D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D020304:Raptors / Q000502:physiology*; D012621:Seasons; D018531:Spacecraft
keywords: animal migration; annual survival; ecology of death; long-distance migration; satellite radio-telemetry